Remove reference to paths ending with backslashes, make more platform-agnostic (dotnet#5419)

* Make docs match how it works on non-Windows

On Windows, this does end with a backslash. But on Linux, it returns `/tmp/`, which ends in a forward slash.
Really, this should read that it ends in DirectorySeparatorChar

* Update xml/System.IO/Path.xml GetFullPath

Add reference to the other field

Co-authored-by: Anton Firszov <[email protected]>

Co-authored-by: Anton Firszov <[email protected]>

Author: Hex052

xml/System.IO/Path.xml

@@ -1906,7 +1906,7 @@ Paths are resolved by calling the <xref:System.IO.Path.GetFullPath%2A> method be
       <Parameters />
       <Docs>
         <summary>Returns the path of the current user's temporary folder.</summary>
-        <returns>The path to the temporary folder, ending with a backslash.</returns>
+        <returns>The path to the temporary folder, ending with a  <see cref="DirectorySeparatorChar"/>.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[